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Switch on to fault (SOTF-OC)
The switch-onto-fault function (SOTF-OC) is used to detect and rapidly clear faults when
circuit breakers (CBs) are closed onto a pre-existing fault in the protected zone. If a circuit
breaker is closed onto a permanent fault, the memory circuit in the IED will be unable to
memorize the voltage data required to retain the pre-fault values. This is because the voltage
input to distance measuring elements is absent continuously prior to and after the fault
particularly with voltage transformers installed on the line side; as a result, the fault cannot
be detected by conventional standard protection functions. Therefore, in order to avoid such
failures, SOTF-OC is applied so that the fault can be detected for a specific period following
the closure of the CB.
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Note: Several IED models do not have switch-on-to-fault features. To determine whether the
features are implemented in the IED or not, locate the IED ordering number; and check the
order number at “G&T position” referring to the comparison table below. For more information,
see
Appendix: Ordering
.
Table 2.14-1 Comparative table in respective ordering numbers
Switch on to fault protection
✓: Applicable NA: Not applicable
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